Carbamic ester derivatives of the general formula (1) and especially (2-carboxy-4-methylphenyl)carbamic esters of the general formula (1′)
are suitable intermediates for active pharmaceutical ingredients.
Thus, for example, hexadecyl (2-carboxy-4-methylphenyl)carbamate as compound of the formula (1′) with R═C16H33 is disclosed as an intermediate in the preparation of 2-hexadecyloxy-6-methyl-4H-3,1-benzoxazin-4-one of the formula (3)
from the originally published version of WO-A 00/40569. 2-Hexadecyloxy-6-methyl-4H-3,1-benzoxazin-4-one of the formula (3) is described therein as potential active ingredient for the treatment of obesity and type II diabetes. In this originally published version of WO-A 00/40569, two synthetic routes 1 and 2 are described for preparing 2-hexadecyloxy-6-methyl-4H-3,1-benzoxazin-4-one (3), each of which starts from the 5-methyl-substituted anthranilic acid (4).
In the two-stage synthetic route 1, the 5-methyl-substituted anthranilic acid (4) is reacted with hexadecyl chloroformate (5) and subsequently with methyl chloroformate to give 2-hexadecyloxy-6-methyl-4H-3,1-benzoxazin-4-one (3), although the overall yield obtained is only 31%.
The one-stage synthetic route 2 with an excess of pyridine affords 2-hexadecyl-oxy-6-methyl-4H-3,1-benzoxazin-4-one (3) in an even lower yield of 15%.

The starting compound which is required for both the synthetic routes 1 and 2, the 5-methyl-substituted anthranilic acid (4), is not easily obtainable, however.
It is prepared by the method described in J. Org. Chem. 1952, 17, 141. This starts from p-toluidine, which is reacted with chloral hydrate and hydroxylamine hydrochloride. The resulting oxime is cyclized with acid catalysis, and subsequently the ring is cleaved again by oxidation under basic conditions.

The disadvantages of this synthesis are the low yields and the fact that only very low concentrations can be used. For this reason, this synthetic route is unattractive for an industrial reaction.
Further alternative routes known in principle for obtaining anthranilic acids are as follows:                J. Org. Chem. 1978, 43, 220 and Chem. Ber. 1909, 42, 430 disclose initial nitration of 3-cyanotoluene, then reduction of the nitro group and subsequent hydrolysis of the nitrile to the carboxylic acid.        
                                    A disadvantage of this synthesis is that the nitration of 3-cyanotoluene does not proceed selectively and therefore a further purification step is necessary. This requires additional effort and reduces the yield.                        The synthesis which is described in J. Chem. Soc. Perkin I, 1973, 2940 and which starts from 3-toluic acid with subsequent nitration and reduction of the nitro group also has the same disadvantage.        The synthesis which is disclosed in Monatsh. Chem. 1920, 41, 155 and starts from 2,4-dimethyl-1-nitrobenzene is likewise unsuitable because oxidation of the methyl group next to the nitro group does not proceed selectively and therefore an elaborate separation of isomers is necessary.        
                EP-A 0 034 292 discloses a process for preparing optionally substituted anthranilic acids which includes a transition metal-catalysed carbonylation reaction with carbon monoxide to give an anthranilic acid derivative. This carbonylation reaction takes place in an aqueous reaction medium containing a trialkylamine and a catalyst formed from palladium and a tertiary phosphine. The anthranilic acid derivatives can be obtained by eliminating the protective group. The precursors employed for the carbonylation are obtained starting from optionally substituted anilines as shown in principle in the reaction scheme below:        
                                    EP-A 0 034 292 describes this reaction sequence of acetylation (a), halogenation (b), carbonylation (c) and subsequent elimination of the acetyl group (d) as affording the optionally substituted anthranilic acids in good yields (>80%). However, the introduction of the acetyl group is a disadvantage. This is necessary because the free anilines give only poor yields in transition metal-catalysed carbonylation reactions because of pronounced complexation [J. Org. Chem. 1981, 46, 4614-4617].                        WO-A 97/28118 discloses a comparable process.        
Because of the diverse difficulties, described above, associated with the known processes for preparing optionally substituted anthranilic acids and the yields, which are only unsatisfactory and thus limiting for the overall process, of the subsequent synthetic routes 1 and 2, the object of the present invention was to provide an improved process for preparing carbamic ester derivatives of the general formula (1).